The present invention is directed to novel chimeric VEGF receptor proteins comprising amino acid sequences derived from the vascular endothelial growth factor (VEGF) receptors flt-1, KDR and the murine homologue of the human KDR receptor, FLK-1, wherein said chimeric VEGF receptor proteins bind to VEGF and antagonize the endothelial cell proliferative and angiogenic activity thereof. The present invention is also directed to nucleic acids and expression vectors encoding these chimeric VEGF receptor proteins, host cells harboring such expression vectors, pharmaceutically acceptable compositions comprising such proteins, methods of preparing such proteins and to methods utilizing such proteins for the treatment of conditions associated with undesired vascularization.
The two major cellular components of the mammalian vascular system are the endothelial and smooth muscle cells. Endothelial cells form the lining of the inner surface of all blood vessels in the mammal and constitute a non-thrombogenic interface between blood and tissue. Therefore, the proliferation of endothelial cells is an important component for the development of new capillaries and blood vessels which, in turn, is a necessary process for the growth and/or regeneration of mammalian tissues.
One protein that has been shown to play an extremely important role in promoting endothelial cell proliferation and angiogenesis is vascular endothelial growth factor (VEGF). VEGF is a heparin-binding endothelial cell growth factor which was originally identified and purified from media conditioned by bovine pituitary follicular or folliculostellate (FS) cells. Ferrara and Henzel, Biochem. Biophys. Res. Comm. 161:851-858 (1989). VEGF is a dimer with an apparent molecular mass of about 46 kDa with each subunit having an apparent molecular mass of about 23 kDa. Human VEGF is expressed in a variety of tissues as multiple homodimeric forms (121, 165, 189 and 206 amino acids per monomer), wherein each form arises as a result of alternative splicing of a single RNA transcript. VEGF121 is a soluble mitogen that does not bind heparin whereas the longer forms of VEGF bind heparin with progressively higher affinity.
Biochemical analyses have shown that VEGF exhibits a strong mitogenic specificity for vascular endothelial cells. For example, media conditioned by cells transfected by human VEGF cDNA promoted the proliferation of capillary endothelial cells, whereas medium conditioned by control cells did not. Leung, et al., Science 246:1306 (1989). Thus, VEGF is known to promote vascular endothelial cell proliferation and angiogenesis, a process which involves the formation of new blood vessels from preexisting endothelium. As such, VEGF may be useful for the therapeutic treatment of numerous conditions in which a growth-promoting activity on the vascular endothelial cells is important, for example, in ulcers, vascular injuries and myocardial infarction.
In contrast, however, while vascular endothelial proliferation is desirable under certain circumstances, vascular endothelial proliferation and angiogenesis are also important components of a variety of diseases and disorders including tumor growth and metastasis, rheumatoid arthritis, psoriasis, atherosclerosis, diabetic retinopathy, retrolental fibroplasia, neovascular glaucoma, age-related macular degeneration, hemangiomas, immune rejection of transplanted corneal tissue and other tissues, and chronic inflammation. Obviously, in individuals suffering from any of these disorders, one would want to inhibit, or at least substantially reduce, the endothelial proliferating activity of the VEGF protein.
In the specific case of tumor cell growth, angiogenesis appears to be crucial for the transition from hyperplasia to neoplasia and for providing nourishment to the growing solid tumor. Folkman, et al., Nature 339:58 (1989). Angiogenesis also allows tumors to be in contact with the vascular bed of the host, which may provide a route for metastasis of tumor cells. Evidence for the role of angiogenesis in tumor metastasis is provided, for example, by studies showing a correlation between the number and density of microvessels in histologic sections of invasive human breast carcinoma and actual presence of distant metastasis. Weidner et al., New Engl. J. Med. 324:1 (1991). Thus, one possible mechanism for the effective treatment of neoplastic tumors is to inhibit or substantially reduce the endothelial proliferative and angiogenic activity of the VEGF protein.
The endothelial proliferative activity of VEGF is known to be mediated by two high affinity tyrosine kinase receptors, flt-1 and KDR, which exist only on the surface of vascular endothelial cells. DeVries, et al., Science 225:989-991 (1992) and Terman, et al., Oncogene 6:1677-1683 (1991). Both the flt-1 and KDR tyrosine kinase receptors have seven immunoglobulin-like (Ig-like) domains which form the extracellular ligand-binding regions of the receptors, a transmembrane domain which serves to anchor the receptor on the surface of cells in which it is expressed and an intracellular catalytic tyrosine kinase domain which is interrupted by a xe2x80x9ckinase insertxe2x80x9d. While the KDR receptor binds only the VEGF protein with high affinity, the flt-1 receptor also binds placenta growth factor (PLGF), a molecule having significant structural homology with VEGF. An additional member of the receptor tyrosine kinases having seven Ig-like domains in the extracellular ligand-binding region is FLT4, which is not a receptor for either VEGF or PLGF, but instead binds to a different ligand; VH1.4.5. The VH1.4.5 ligand has been reported in the literature as VEGF-related protein (VRP) or VEGF-C.
Recent gene knockout studies have demonstrated that both the flt-1 and KDR receptors are essential for the normal development of the mammalian vascular system, although their respective roles in endothelial cell proliferation and differentiation appear to be distinct. Thus, the endothelial proliferative and angiogenic activity of the VEGF protein is mediated by binding to the extracellular ligand-binding region of the flt-1 and KDR receptors on the surface of vascular endothelial cells.
In view of the role of VEGF in vascular endothelial proliferation and angiogenesis, and the role that these processes play in many different diseases and disorders, it is desirable to have a means for reducing or inhibiting one or more of the biological activities of VEGF. As such, the present invention is predicated upon research intended to identify the Ig-like domain or domains of the flt-1 and KDR receptor extracellular ligand-binding region which mediate binding to the VEGF protein and inserting or fusing that domain or domains into amino acid sequences derived from another protein to produce a xe2x80x9cchimeric VEGF receptor proteinxe2x80x9d. The chimeric VEGF receptor proteins of the present invention will bind to and inactivate endogenous VEGF, thereby providing a means for reducing or inhibiting endogenous VEGF activity and, in turn, reducing or inhibiting endothelial cell proliferation and angiogenesis. Thus, it is an object of the present invention to provide novel chimeric VEGF receptor proteins comprising amino acid sequences derived from the extracellular ligand-binding region of the flt-1 and KDR receptors, wherein said chimeric VEGF receptor proteins are capable of binding to and inhibiting the activity of VEGF.
Further objects of the present invention are to provide nucleic acids encoding chimeric VEGF receptor proteins of the present invention, replicable expression vectors capable of expressing such chimeric proteins, host cells transfected with those expression vectors, pharmaceutical compositions comprising the chimeric VEGF receptor proteins of the present invention, methods for preparing such chimeric proteins and method of using those chimeric proteins for the therapeutic treatment of an individual in need thereof.
The objects of this invention, as generally defined supra, are achieved by the provision of chimeric VEGF receptor proteins which are capable of binding to VEGF and exerting an inhibitory effect thereon, wherein said chimeric VEGF receptor protein comprises Ig-like domains 1, 2 and 3 of the flt-1 and/or the KDR receptor (or the murine homologue of the KDR receptor, FLK-1) or functional equivalents thereof.
In a preferred embodiment, the chimeric VEGF receptor proteins of the present invention contain flt-1 or KDR receptor amino acid sequences corresponding only to Ig-like domains 1, 2 and 3 of the extracellular ligand-binding region thereof and each Ig-like domain is derived from the same VEGF receptor.
In other embodiments, however, the chimeric VEGF receptor proteins of the present invention comprise Ig-like domains 1, 2 and 3 of the extracellular ligand-binding region of the flt-1 or KDR receptor in addition to one or more of the remaining four immunoglobulin-like domains thereof. Preferably, the Ig-like domains employed are derived from the same receptor, however, a combination of Ig-like domains derived from both the flt-1 and KDR receptors will find use.
In another embodiment of the present invention, the chimeric VEGF receptor proteins of the present invention comprise the extracellular ligand-binding region of the FLT4 receptor wherein at least Ig-like domain 2 of the FLT4 receptor is replaced with the Ig-like domain 2 of either the flt-1 or KDR receptor. Preferably, only Ig-like domain 2 of the FLT4 receptor is replaced by the corresponding Ig-like domain from either the flt-1 or KDR receptor, however, other domains may also be similarly replaced.
A further aspect of the present invention is directed to nucleic acid sequences encoding the chimeric VEGF receptor proteins described herein and functional equivalents thereof. It is well known to the ordinarily skilled artisan that such nucleic acids can vary due to the degeneracy of the genetic code and such nucleic acid variants are also encompassed by the present invention.
In still other embodiments, the present invention relates to replicable expression vectors encoding the various chimeric VEGF receptor proteins described supra, host cells transfected with those expression vectors and compositions comprising the chimeric VEGF receptor proteins described supra compounded with a pharmaceutically acceptable excipient.
In yet other embodiments, the present invention relates to methods for producing the chimeric VEGF receptor proteins described supra by introducing an expression vector encoding the desired chimeric protein into an appropriate expression systems and effecting the expression of said protein.
Yet another aspect of the invention provides for the use of the chimeric VEGF receptor proteins of the present invention for the treatment of conditions associated with inappropriate vascularization wherein an inhibition of vascularization and angiogenesis is desirable.